1. Field of the Invention
This invention relates to strain gage load cells used for weighing heavy loads, and to weighing systems using the load cells.
2. Description of the Related Art
Strain gage load cells are used for weighing of trucks and railway cars, and for weighing of hoppers and tanks in the processing industry. A platform or a frame adapted to carry the load to be weighed is supported on a plurality of load cells resting on base plates below the platform or a frame, and the signals from the individual load cells are added electronically to obtain a signal or indication proportional to the total weight to be measured.
A load cell can measure a force in its sensing direction with high accuracy, but lateral forces and eccentric loading will cause errors in the output signal. Lateral forces and eccentric loading are introduced when a truck drives onto and brakes or accelerates on a weighing platform, when temperature changes due to direct sunlight and/or the ambient temperature cause thermal expansion of the platform or frame, and when the platform or frame deflects under load.
Ever since strain gage load cells were first used for weighing, more than three decades ago, designers and inventors have tried to minimize the detrimental effect of lateral forces and eccentric loading.
U.S. Pat. No. 2,962,276 describes a platform scale using compression load cells in the form of columns with spherical end surfaces fitted in matching spherical sockets in the platform and in the support structure. Membranes are provided to maintain the relative position of the platform and the support structure. The spherical sockets will provide compensation for misalignments during installation, and the membranes will contain the side forces caused by a truck entering the platform. Sliding friction between the end surfaces of the load cells and the matching spherical sockets will, however, cause bending stresses in the load cells whenever there is a change in relative position between the platform and the support structure. Thermal expansion and deflection of the platform under load cause such position changes, with corresponding errors in the load cell output signal that cannot be corrected. The scale shown in this patent accordingly cannot provide accurate weighing under normal operating conditions.
U.S. Pat. No. 3,164,014 describes a load cell where a compression column is mounted in a rigid housing with membranes shunting side forces away from the measuring section of the load cell. In addition, the strain sensors are positioned at a point where the effects of eccentric loading is minimized. This type of load cell works quite well as long as the side forces are moderate, but it must be loaded via roller bearings or other devices designed to remove the lateral forces caused by dimension changes when used with a large platform installed outdoors unprotected from environmental temperature changes. This adds to the cost and the complexity of the overall scale.
U.S. Pat. No. 3,736,998 describes a force application device designed for the type of load cell described in U.S. Pat. No. 3,164,014. The device acts as a swing, allowing movement of the platform relative the load cell with only small residual side forces on the load cell. This combination works very well, and has been widely used commercially, but it is an expensive solution.
U.S. Pat. No. 4,162,628 is an alternative to the load cell according to U.S. Pat. No. 3,164,014, but it utilizes a shear sensing transducer instead of a compression column, which reduces the sensitivity to lateral forces and eccentric loading. Even this type of load cell cannot handle large dimensional changes directly, but it could possibly use teflon lubricated sliding plates instead of roller bearings.
Swedish Pat. No. 366 116 describes a combination of a load cell with special properties and a self-stabilizing rocking device. The load cell is designed to have a sensitivity which increases linearly when the load application center is moved away from the symmetry center, and decreases linearly when the angle of the resultant force deviates from the vertical. The patent describes how the measuring error can be reduced to near zero in this combination by selecting the radii and the height of the rocking device properly. The patent also states that such a load cell can be an integral part of the rocking device, but it does not disclose any such integrated design. It is doubtful if an integrated design will work with acceptable accuracy in practice, because there will not be any force component at right angle to the measuring axis in the described load cell when the load cell is integrated with the rocker device, and there will accordingly not be any error compensating effect as required by the patent. The fact that the patent owner has marketed only the separate load cell/rocker body combination, but never as an integrated design is further evidence that the integrated design does not work.
U.S. Pat. No. 4,248,317 describes an alternative to the swing device of U.S. Pat. No. 3,736,998. A compression type load cell, similar to U.S. Pat. No. 3,164,013 or U.S. Pat. No. 4,162,628, is mounted in a rigid housing with a membrane on top. A loading member with a spherical top surface in contact with a flat loading surface is mounted on the membrane, and the bottom of the housing is provided with another spherical surface in contact with a flat support surface. If the loading platform moves, or if its dimensions change relative the support structure, the load cell housing will roll essentially friction free between the two flat surfaces. The rolling will, however, make the axis of the load cell tilt from the vertical, which gives rise to a force component twisting the loading member on top of the load cell so the membrane will become S-shaped. The flexing of the loading member will cause a lateral force component on the load cell, with undesirable errors in the output signal, contrary to the theory of the patent. This design is also both expensive and bulky.